In gas metal arc welding processes, there is provided a feed wire electrode. An electrical arc is struck between this electrode and the work to be welded and normally an inert gas is provided within an appropriate shroud in the arc environment. There may be traces of chemically active gases in this inert gas.
The feed wire electrode is continuously fed towards the work and is consumed by melting under the intense heat of the arc, the metal of this electrode thus depositing on base material and in forming the weld.
With respect to the foregoing, a most desirable type of metal transfer mode is known in the art as "spray transfer." In this type of transfer, the current density and resultant high coaxial magnetic field in combination with a pressure, voltage gradient between electrode tip (wire) and base metal, results in molten metal particles being ejected from the end of the feed wire as it is being continuously fed towards the work. These particles are transferred across the arc and impinge on a molten puddle which is formed in the work. Unless this puddle is properly controlled, it can become sufficiently large that it is not possible to maintain a spray transfer mode for out-of-position welding applications; for example, in welding about the circumference of a pipe wherein the gravitational components causes undue flowing of the weld puddle.
If the curent density is reduced, there is reached a value which is not sufficient to expel the metal from the end of the feed wire and the metal will be melted and form a droplet. The droplet itself increases in size until its mass causes it to break loose and fall toward the puddle. Impacting of the puddle by the drop will splash molten metal around the weld zone. Such turbulent action can result in cold lapping, excessive spatter accumulation on the torch and surrounding work and a poor quality weld.
If still less current density is utilized, the feed wire electrode will move towards the molten puddle faster than droplets can be formed. As a consequence, no metal transfer takes place until physical contact is made between the droplet starting to form on the end of the feed wire and the puddle. At this point, a short circuit is established extinguishing the arc and a massive current surge is initiated by the normally provided constant voltage power supply. Within a short time, the feed wire is fused until it becomes molten.
The foregoing fusing action re-establishes the arc and the current returns to its original low power level which reinitiates the entire process. This action can occur at frequencies of up to 200 Hz. The action is known in the art as a short circuiting arc process. Because the molten puddle under this short circuiting arc process is smaller, it is capable of useful operation in out-of-position welding. On the other hand, the deposition rates are low for the short circuiting process and in general poor weld quality due to porosity and cold lapping often result.
Some of the foregoing difficulties have been overcome with the advent of semi-conductor technology by utilizing what is known as a pulsed spray transfer mode for out-of-position welding. Essentially, the power supply output was pulsed at a frequency of either 60 or 120 Hz. Under these conditions, the power supply would output a DC base power level adjusted to a value just sufficient to keep the arc ignited. Then at 60 or 120 times per second a very high power level pulse would be superimposed on the base level. The high current density produced by this pulse would eject metal particles in a spray transfer mode. When the pulse was removed, the metal transfer was terminated and a lower power arc would be maintained until the next pulse.
The foregoing arrangement essentially permitted some puddle control by alternate cooling so that out-of-position welding could be achieved. Further, many of the deficiencies characteristic of the short circuiting arc process were overcome although there still resulted a relatively low deposition rate.
The basic problem in all instances is that of arc stability. If there were some means of providing for a stabilized arc, then various further control processes could readily be carried out, all to the end that puddle control and deposition rates on a work in out-of-position welding could be maximized.